We used holes defined by binocular disparity to study how the inner contour of an object (i.e., the boundary of a hole) is encoded in visual working memory (VWM). Many studies in VWM have shown that an object's contour properties can be integrated with its surface properties via their shared spatial location, yielding an object-based encoding benefit. However, encoding of the hole contours has rarely been tested.

We presented circles containing a bar under a change detection paradigm. To compare VWM capacity for objects having a hole to those with the corresponding solid complement, we manipulated binocular disparity of bars, creating two types of displays, a hole display and a conjunction display. Relevant features to be remembered were the color of circles and the orientation of bars (or holes).

If the contour defining a hole is perceptually owned by the surrounding object (an outer circle) rather than by the hole itself, the object-based encoding hypothesis (Luck & Vogel, 1997) predicts that the orientation can be integrated with the color of an outer circle via their shared spatial location. Thus, in the hole display, change detection performance should be better than in the conjunction display where orientation and color were assigned to different parts of an object, and comparable to performance in a single bar display where both orientation and color were assigned to a single bar. However the results revealed that performance in the hole display did not differ from that in the conjunction display, suggesting that the shape of a hole is encoded with the same load as that of its complement. We conclude that the boundary defining a hole is not automatically encoded together with the surface properties of the outer object via object-based feature binding, but rather is encoded independently from the surrounding object.